Gas turbine engine components that form part of the hot gas path are exposed to extremely high operating temperatures and stresses, and the temperatures and stresses continue to rise as technology improves and lower emissions and higher efficiencies are required. Several methods exist for creating hot gas path components such as vane segments or blades. Vane rings, for example, may be a single piece for smaller configurations, or may be composed of multiple vane segments in larger configurations. Vane segments may, in turn, be composed of multiple airfoils joined at one end by an outer ring segment, and at the other end by an inner ring segment, or a single airfoil with an outer ring segment and an inner ring segment.
Different materials and structures are known for use in vane segments. For example, monolithic airfoil segments made of polycrystalline superalloys have been used. Polycrystalline superalloy structures have good heat resistance, good mechanical properties, and good oxidation and corrosion resistance. However, modern gas turbines are exceeding the capacity of even these materials. One solution has been to produce monolithic vane segments using a single crystal superalloy. Single crystal superalloys offer exceptional mechanical properties (strength, fracture toughness, fatigue) and a good balance of mechanical properties, heat resistance, and oxidation and corrosion resistance, and are thus well suited for airfoil applications. However, the process for making a monolithic single crystal superalloy vane segment is challenging and unacceptable casting defects frequently result in low part yields, thereby making such parts very expensive. The casing difficulties limit larger sized vane segments to a single airfoil. It is also known in the art to assemble multiple airfoils in a jig and join them together via a casting operation. In this method, commonly referred to as bi-casting, the airfoils are a single type of preformed component and are joined by one material. Other methods for manufacturing vane segments include making an airfoil from more than one component of dissimilar materials and then brazing the assembly to the shroud segments.
However, there is room for improvement with these methods because they retain negative characteristics in terms of their performance, their cost, and/or the performance of the hot gas flowing there through.